Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 6,952.2	$ 6,284.7
Equity securities	315.8	246.4
Other investments	210.3	190.2
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	172.8	138.7
Total investment assets at fair value	7,431.1	6,660.4
Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	306.1	237.0
Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,952.2	6,284.7
Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	325.6	269.3
Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	352.9	239.0
Fair Value, Measurements, Recurring | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,096.3	1,028.0
Fair Value, Measurements, Recurring | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,773.4	3,375.6
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities U S Agency Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	610.8	663.3
Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	194.4	185.3
Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	396.2	398.1
Fair Value, Measurements, Recurring | Fixed Maturities | Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	202.6	126.1
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investment assets at fair value	432.0	324.7
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	226.9	177.4
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	205.1	147.3
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	144.2	147.3
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	60.9
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	169.2	135.1
Total investment assets at fair value	6,896.4	6,233.2
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	54.8	36.2
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	6,672.4	6,061.9
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | U.S. Treasury And Government Agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	181.4	122.0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Foreign Governments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	292.0	239.0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	1,076.9	1,014.4
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	3,747.0	3,351.8
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage Backed Securities U S Agency Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	610.8	663.3
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	193.7	180.1
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	369.5	374.4
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Fixed Maturities | Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	201.1	116.9
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments	3.6	3.6
Total investment assets at fair value	102.7	102.5
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities	24.4	23.4
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	74.7	75.5
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	19.4	13.6
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	26.4	23.8
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Residential Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	0.7	5.2
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Commercial Mortgage-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	26.7	23.7
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Fixed Maturities | Asset-Backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities	$ 1.5	$ 9.2